1. Technical Field
The invention is generally related to the field of extensometers and to the field of devices that measure the extension or strain in a specimen subjected to tension, compression, flexure, impact or other loading conditions. The invention also relates to extensometers that can be used across cracks and on surfaces with a high degree of roughness in a specimen.
2. Related Art
An extensometer measures the change of distance between two points on a specimen by attaching its two extension legs to the specimen and then activating the extensometer. FIG. 1 illustrates a known extensometer. Some problems in current extensometers include the generally large size of current devices, which can induce stresses in the specimen, and the occurrence of knife-edge slip, which can lead to incorrect measurements. In addition, many current extensometers are supported on the specimen itself, which can produce substantial bending of the specimen, or at least induce bending stresses. Such bending or bending stresses generally are not acceptable, and are especially unacceptable in biological and soft materials. Known non-contact extensometers, for example laser extensometers, may eliminate the slip problems but they are high cost instruments and can be used only on certain types of materials, specimen shapes, and/or surface finishes. Most commercial extensometers are able to take measurements in only one direction (either tension or compression) or only in a plane.
Some Fiber Bragg Grating (FBG) fiber-optic based extensometers have been developed for use in structural health monitoring. Yet the large size, diligent setup, and low sensitivity and dynamic range of these FBG extensometers are disadvantages. FBG extensometers also generally cannot be used on small diameter specimens, crack opening type measurements, and irregular surfaces.
Common strain gauges also suffer from the same disadvantages mentioned above and demand perfect bonding with the surface and high degree of surface finish. In addition, measurements obtained from these strain gauges strongly depend on the adhesive used in bonding them with the specimen.
Fiber-optic microbend sensors suffer from the disadvantage of having lower relative durability as the fiber is curved in micrometer radius. The microbend sensors also have a larger size, which is not appropriate in embedding inside materials or structures for structural health monitoring.
U.S. Pat. No. 7,174,061 B2 to Rougeault et al. discloses a known extensometer to measure the deformations of a host material. This extensometer comprises at least one test specimen and at least one Bragg grating formed in an optic fiber which is made integral with the test specimen. Any deformation of the host material is transmitted to the Bragg grating and the Bragg grating is configured to then being able to modulate the light propagating in the fiber. The deformation of the host material is determined by calibrating the modified light with respect to known forces and deformation. The test specimen undergoes linear bending stresses while remaining within its range of elastic deformation. This extensometer also comprises mechanical means able to transform deformation of the host material into bending of the test specimen, which deforms the Bragg grating. The mechanical means comprises a first part, intended to be made rigidly integral with the host material and in which the test specimen is fixed, and a second part also intended to be made rigidly integral with the host material and which is able to move within the first part and to cause bending of the test specimen.
U.S. Pat. No. 6,956,981 B2 to Dewynter-Marty et al. discloses another known extensometer comprising an optical fiber, in which at least one Bragg grating is formed, and at least one proof body configured to be rigidly fixed to a host material and that surrounds part of the optical fiber containing the Bragg grating. Any deformation of the host material is transmitted to the Bragg grating through the proof body. The Bragg grating is configured to modify a light propagating in the optical fiber. Any deformation of the host material is determined from the modified light. The proof body comprises a tube in which the part of the optical fiber containing the Bragg grating is placed, with two ends of the part of the optical fiber being fixed to corresponding two ends of the tube and the part of the optical fiber is tensioned between the two ends of the tube.
U.S. Pat. No. 5,090,248 to Cimmino et al. discloses another known extensometer for measuring dimensional change. This extensometer comprises two or more adjacent electrical conductors selected and configured to allow relative positioning changes thereof to cause a change in electrical interaction between said conductors. The conductors are thin, pliable, electrically conductive wires wound in the form of interposed helical coils. The coils include at least two adjacent turns that are completely encased in an elastic dielectric material as a principal means for restoring the wires to their original configuration after a positioning change thereof. The configured electrically conductive wires and the restorative elastic material combined facilitate accurate conformability during use and accurate measurement of small and substantial displacements, extensions, dilation, and torsions about a longitudinal extent of the extensometer.
U.S. Pat. No. 5,258,614 to Kidwell et al. discloses a known fiber optic loop temperature sensor, comprising a first multi-mode optical fiber being formed with a plurality of loops, a light source, a second reference optical fiber, and optical detecting means. The loops each have a predetermined radius fixed with securing means. The first optical fiber is positioned in an environment in which a temperature is to be measured. The light source is for supplying light in only one direction to said signal optical fiber. Each of the loops changes in diameter as a function of temperature to effect changes in light transmission therethrough. The second reference optical fiber receives light from the light source for providing a reference light intensity. The optical detecting means is for measuring light intensities from both optical fibers to determine the temperature from differences therein.
Current mechanical or laser extensometers, strain gauges, Fiber-Bragg gratings, and fiber-optic interferometers are used for displacement or strain measurement. However, the sensitivity of such current mechanical extensometers is much lower than that of optical extensometers. Additionally, such current mechanical extensometers tend to be bulky. Accordingly, there is always a need for an improved extensometer. Additionally, there is always a need for an improved extensometer that does not cause undue stress on or bending of the sample. Further, there is always a need for extensometers that can be more easily attached to samples and that do not necessarily require a high degree of surface finish. It is to these needs, among others, that this invention is directed.